Wire GitHub's claude-code-action without granting it more authority than the job needs

Write a GitHub Actions workflow YAML using anthropics/claude-code-action for this specific automated job, scoped as narrowly as the job allows — an unattended run in CI has no human present to answer a permission prompt, so anything left too broad here executes silently rather than pausing for confirmation.

WHAT SHOULD TRIGGER THIS WORKFLOW
An issue_comment or pull_request_review_comment containing the exact string @claude, on issues and PRs only, not on every push.

WHAT THIS WORKFLOW IS ACTUALLY RESPONSIBLE FOR
Read the PR diff and post a structured review comment back to the PR; it must never push a commit or modify a file directly.

GITHUB PERMISSIONS THIS JOB NEEDS
contents: read, pull-requests: write (to post the review comment), nothing else.

SECRETS THIS JOB REQUIRES
ANTHROPIC_API_KEY, stored as a repository secret, not an organization-wide one.

RUNNER ENVIRONMENT
ubuntu-latest, no dependency installation needed since this job only reads the diff via the GitHub API.

PRODUCE
1. The trigger block, matching An issue_comment or pull_request_review_comment containing the exact string @claude, on issues and PRs only, not on every push. precisely — if this should only fire on an @claude mention inside an issue or pull-request comment, scope it to that event and that exact string, not a broader event type that would also fire on unrelated activity and consume budget or take action nobody asked for.
2. The permissions block for GITHUB_TOKEN, granting only what contents: read, pull-requests: write (to post the review comment), nothing else. actually requires — contents: read if the job only reads code, pull-requests: write only if it must actually leave a comment or push a commit, never permissions: write-all as a default to avoid enumerating the real list.
3. The claude-code-action step itself, with allowed-tools scoped to Read the PR diff and post a structured review comment back to the PR; it must never push a commit or modify a file directly. — a job whose responsibility is reviewing a diff and posting a comment should not carry Bash access broad enough to also modify files, even though the underlying action supports it, because a review job that can silently also edit code is no longer just a review job.
4. The secrets reference for ANTHROPIC_API_KEY, stored as a repository secret, not an organization-wide one., pulled from repository or organization secrets, never hardcoded in the workflow file, and a note on which specific secret this job actually needs versus which secrets exist in this repository more broadly that it should not have access to.

CONSTRAINTS
- If Read the PR diff and post a structured review comment back to the PR; it must never push a commit or modify a file directly. indicates this job should never push a commit or modify a file directly, do not grant contents: write or Edit/Write tool access even if a future version of this job might want it — scope for what this job does today, and revisit the scope explicitly when the job's responsibility actually changes, rather than provisioning ahead of need.
- State what happens if An issue_comment or pull_request_review_comment containing the exact string @claude, on issues and PRs only, not on every push. fires on a pull request from a fork — a workflow with write permissions triggered by an external, untrusted contributor's PR is a materially different risk than the same workflow triggered only by a maintainer's own comment, and this distinction needs to be addressed explicitly, not left to whatever GitHub's default behavior happens to be.
- Confirm ubuntu-latest, no dependency installation needed since this job only reads the diff via the GitHub API. matches what this job actually needs — a job that only calls the GitHub API and posts a comment does not need the same runner image as one that installs dependencies and runs a full build.

OUTPUT
The complete workflow YAML, followed by one paragraph stating exactly what this workflow can and cannot do to this repository, written for a reviewer approving it for the first time.